The present invention relates to an hydraulic control system, particularly for a motor vehicle braking apparatus, having at least one hydraulic braking circuit, the hydraulic control system being of the kind comprising a control unit, a brake pedal, an assisting hydraulic pressure plant having a fluid pressure source and a reservoir, a distributor valve having a slide fitted movably in the control unit between a rest position in which it places the braking circuit in communication with the reservoir, and an active position in which it places the braking circuit in communication with the fluid pressure source, said slide being subject to two opposing forces, one of which is developed by the pedal and tends to bring it into the active position, whereas the other is developed by the pressure of the said at least one braking circuit and tends to return it into the rest position, first resilient stroke simulator means being interposed between the pedal and the slide, whereas second resilient means are interposed between the pedal and the unit.
In hydraulic control systems of this type, the movement stroke of the slide of the distributor valve to increase or decrease the hydraulic pressure in the braking circuit as a function of the force exerted by the driver on the pedal, which force is a measure of the driver's intention to increase or decrease the braking, is very small, e.g. of the order of one millimeter. This is the reason why the pedal stroke simulator means interposed between the pedal and the slide are provided. In this way the driver is able to displace the pedal with his foot through a stroke which is not of the order of a millimeter, but the value of which is more appropriate for driving convenience, that is to say of the order of ten centimeters, which gives the driver total control of braking, with a feeling similar to that which is obtained with an unassisted control, and with greater efficacity.
The second resilient means interposed between the pedal and the unit generally has hitherto had the function of a simple resilient return means, and customarily acts all along the stroke of the pedal.
In apparatuses of this type, which have the advantage of enabling good control of a braking system with a smaller muscular effort, it is sometimes found that this very facility can lead to excess. In fact the pressure of the hydraulic plant must be made stronger than necessary for ordinary requirements, to allow for the reduction in the efficiency of the brakes when the latter are hot. In the case of danger, even an experienced driver may press the pedal too suddenly, which, because of the pressures developed by the hydraulic plant, may cause locking of the wheels, or even rupture of the braking circuit.
The object of the present invention is to provide an hydraulic control system of the kind hereinbefore described, which is free of this disadvantage and which enables, with a simple and robust construction, excellent braking conditions without the risk of harmfully excessive pressure, whilst giving the driver the possibility of developing good braking pressure even when the brakes are very hot.